1. Field of the Invention
The present invention relates to a packing material for liquid chromatography, comprising a spherical clay mineral powder which comprises a naturally occurring or synthetic water-swellable clay mineral, a water-swellable clay mineral having foreign cations substantially substituted for interlayer ions inherently present therein, or a calcined product thereof.
2. Description of the Related Art
From the viewpoint of the separation mechanism, liquid chromatography is generally classified into adsorption chromatography and partition chromatography, and the latter is further classified into normal phase chromatography and reversed phase chromatography, in view of the combination of the polarity of an eluent and a packing material.
As the packing material for adsorption or normal phase partition chromatography, a packing material based on a porous silica having reticulately distributed macro- and micro-pores is predominantly employed. Further, packing materials based on an alumina or porous polymer such as stylene divinylbenzene or hydroxymethacrylate, or those based on a calcium hydroxide powder or calcium phosphate gel are also used, for limited purposes.
For reversed phase partition chromatography, the packing material having alkyl groups such as an octadecyl group or the like chemically bonded to the above-mentioned packing material such as silica, as a carrier, is widely employed, and a typical example thereof is ODS (octadecylsilane).
With respect to the form of the packing material particle, two types are known, i.e., spherical and irregular particles.
The spherical packing material can be packed at a high density, while ensuring a good reproducibility, and thus exhibits a good column efficiency.
In adsorption and normal phase chromatography, silica gel with a pore size of about 60-100 .ANG. and a specific surface area of about 300-500 m.sup.2 /g is most widely used. The silica gel has silanol groups (Si-OH) as an active site on the surface, and since an adsorption energy of a functional group in a solute molecule against the silanol groups is an important factor in the separation, such a silica gel is limited in application to non-ionic molecules having a small number of functional groups including oxygen or nitrogen (e.g., carbonyl, hydroxyl, or amino group). Preferably, the silica gel packing material could be applied for solutes having a neutral polarity and an intermediate molecular weight, but it is difficult to apply for an ionic or high polar material.
The porous polymer exhibits a special interaction with the solute molecule, regardless of the kind of the polymer used. The porous polymer packing material has a separation mechanism different from that of the silica gel packing material, and in many cases, the porous polymer packing material can separate solute molecules which are difficult to separate by the silica gel packing material. Nevertheless, the porous polymer packing material has a disadvantage in that the separation efficiency is reduced due to dissolution or swelling of the polymer, and further, the pressure resistance of the porous polymer packing material is inferior to that of the silica gel packing material.
In the silica packing material having alkyl groups bonded to the surface thereof and conventionally used for reversed phase partition liquid chromatography, 10-20% of the silanol groups remain unreacted because of an incomplete reaction between the silanol groups and alkyl chlorosilane. The presence of such residual silanol groups is a serious problem in reversed phase partition liquid chromatography. The residual silanol groups, if any, strongly interact with a polar substance, and thus the analysis or purification of the polar substance results in a wide peak and tailing. Further, a highly polar substance irreversibly adsorbs such residual silanol groups, and thus the function of a column packed with such a conventional material rapidly deteriorates. In addition, the presence of a hydrophilic surface of the silica gel as well as residual silanol groups thereon causes a conventional packed column to become weak against acids and alkalis, and thus limits the pH region of a mobile phase solvent to within 2 to 7.
To solve the problems mentioned above, some conventional packing materials prepared by a reaction with dimethyloctadecylchlorosilane or dimethyloctylchlorosilane have been treated again with trimethylchlorosilane (TMS) or the like, to block the residual silanol groups. But, such a post-treatment with TMS cannot completely avoid the presence of residual silanol groups.
Recently, cases wherein an optical resolution of optical active substances is necessary have increased. As the optical resolution methods, there may be mentioned a method making use of the differences of physicochemical properties of chemically produced diastereomers, a method using an enzyme, a method of chromatography, or the like. In the chromatography method, the differences of both optical antipodes with respect to physical or chemical adsorption, or partitioning against a chiral adsorbent, are used. Thus, the open column method is now disregarded, since recently a high performance liquid chromatography for optical resolution has been developed and used.
In the liquid chromatography for optical resolution, a packing material carrying an optically active ligand adsorbed or chemically bonded to the silica gel or organic polymer powder, or a packing material comprising a polymer having an optical activity has been employed. Further, a method was proposed wherein a resoluting agent of a clay mineral prepared by ion-exchanging interlayer ions thereof for ions from an optically active metallic complex was used.
When the above packing material for optical resolution is used, the substances which can be resoluted are limited, due to the packing material used, and in many cases, the resolution cannot be completed. Further, in many cases it is difficult to use the packing material for a long term, due to a durability problem caused by a desorption of the ligand or a swelling of the packing material.
The metallic complex-clay adduct disclosed in Japanese Unexamined Patent Publication No. 61-204138 does not have a uniform particle size or particle size distribution, or regular shapes of the clay mineral, and thus is difficult to use as the packing material for high performance liquid chromatography. Accordingly, since it can be used only batchwise or in an open column, the resolution is relatively ineffective.
Although the adsorbent prepared by attaching the adduct to the silica gel or the like (Japanese Unexamined Patent Publication No. 61-204562) may be used as the packing material for high performance liquid chromatography, it has a low ratio of the metallic complex-clay adduct (which is necessary for optical resolution) to the total packing material, and thus does not provide many adsorbing sites. Therefore, many substances are eluted without being adsorbed, and thus substances to be applied therefor are extremely limited. If only the few substance which may be applied are used, a disadvantage arises in that it is necessary to add water to the eluent to lower the solubility of the substance in the eluent and increase the adsorption thereof to the packing material.